There are many localized disease conditions which are effectively treated by topical application of suitable physiological agents. In order for such treatments to be maximally effective, it is necessary that as much of the pharmacologically active agent as possible be absorbed into the skin where it can make contact with the disease condition in the dermal tissues without being lost by rubbing off on clothing or evaporation. At the same time, the agent must not penetrate so effectively through the skin as to be rapidly lost to the lymphatic and vascular circulatory systems. This latter factor is especially important when the pharmacologically active agent is toxic when used systemically.
The ideal vehicle for topically applied pharmaceuticals is therefore one which can produce a "reservoir effect" in the skin or mucous membranes to which the topical treatment is applied. This "reservoir effect" is defined as an enhancement of the skin or membrane's ability to both absorb and retain pharmacologically active agents, i.e., to increase skin or membrane residence time decrease drug transit time and reduce transdermal flux.
A number of compounds are known to enhance the ability of pharmacologically active agents to penetrate the skin and mucous membranes, for example, N-bis-azacyclopentan-2-onyl-alkanes, 1-substituted azacycloheptan-2-ones and higher alkyl-substituted azacyclopentan-2-ones, as well as dimethylsulfoxide and lower alkyl sulfoxides. These compounds, however, have the disadvantage of allowing rapid systemic dispersion of the pharmacologically active agents away from the localized site of pathology. Many topical medicaments, such as the retinoids used in the treatment of acne, and methotrexate, used in the treatment of psoriasis, are systemically toxic. The retinoids, for example, are known to cause damage to unborn fetuses. Thus, there is a need for a method of enhancing the ability of such medicaments to penetrate into the skin or mucous membrane so that a lesser total dosage may be used, while at the same time retarding their ability to move from the skin to the interior of the body.
The problem posed by the paradoxical requirement that a systemically toxic topical medicament be efficiently absorbed into, but not through, the skin, has heretofore gone unrecognized. There have been no vehicle additives available to the pharmacological industry which act to enhance the "reservoir capacity" of the skin and mucous membranes so that the amount of pharmacologically active agents reaching and being retained at the site of localized pathologies is maximized.